Process for extruding low density foam using water as a blowing agent

Abstract

In a process for extruding a low density foam, a foamable thermoplastic elastomer such as a thermoplastic rubber is selected. The selected thermoplastic elastomer is then fed into a heated extruder barrel where it is compressed and melted by the action of a screw. A blowing agent consisting of water is introduced into the melted thermoplastic elastomer. The melted thermoplastic elastomer and the water are thoroughly mixed and then cooled to a uniform, predetermined temperature. The mixture is then forced through a die to form an extruded foam profile.

We claim: 
     
       1. A method for extruding a thermoplastic elastomer seal comprising the steps of: selecting a foamable thermoplastic elastomer composition comprising a blend of between 25 to 75 percent by weight of a thermoplastic polypropylene resin and, correspondingly, 75 to 25 percent by weight of fully cured monoolefin copolymer rubber;   compressing and heating the thermoplastic elastomer composition in a mixing vessel to a temperature above the melting point of the thermoplastic resin;   introducing into the heated thermoplastic elastomer composition while under pressure within the mixing vessel a blowing agent consisting of water in an amount sufficient to produce closed cell foaming of the thermoplastic elastomer composition;   mixing and heating the thermoplastic elastomer composition including melted thermoplastic resin and the water in the mixing vessel; and   forcing the heated composition and water through a die opening shaped like a cross-section of a seal to form an extruded seal of substantially closed-cell foam thermoplastic elastomer substantially impervious to the passage of fluid therethrough.   
     
     
       2. The method according to claim 1 wherein the thermoplastic elastomer comprises a blend of natural or synthetic rubber and thermoplastic material. 
     
     
       3. The method according to claim 1 wherein the step of introducing water includes introducing the water in sufficient quantity relative to the thermoplastic elastomer to yield a seal having a density below approximately 0.3 grams per cubic centimeter. 
     
     
       4. The method according to claim 1 wherein the mixing vessel comprises an extruder and the thermoplastic elastomer is continuously extruded in the extruder, the extruder having a length-to-diameter ratio above approximately 32:1. 
     
     
       5. The method according to claim 1 wherein the mixing vessel comprises an extruder and the thermoplastic elastomer is continuously extruded in the extruder and further including the step of providing a direct pressure-regulated hook-up from a local water system to the extruder barrel and wherein the step of introducing water employs water from the direct hook-up. 
     
     
       6. The method according to claim 5 wherein the step of providing a direct hook-up further includes filtering the water with a double filter system comprising two valves, and wherein the method further includes the step of closing one of the valves and the step of changing one of the filters of the system.